Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a small volume and does not need to be stored at high pressure.
Usually, natural gas comprises predominantly methane. In addition to methane, natural gas usually includes some heavier hydrocarbons such as ethane, propane, butanes, C5+ hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most if not all known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process.
Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGL) from a natural gas stream, generally termed NGL recovery. The NGLs are usually fractionated to yield valuable hydrocarbon products, either as products steams per se or for use in liquefaction, for example as a component of a refrigerant.
NGL recovery generally involves an NGL separation column in which the natural gas stream is separated into a bottom stream containing the NGLs and a methane-enriched overhead stream, which is often compressed or recompressed (the natural gas stream may have been depressurized upstream of the NGL separation column) by one or more compressors.
Compressors for gaseous streams are used in many situations, systems and arrangements. Usually there is a vapour recycle or recirculation line around the compressor to avoid ‘surge’. Normally, surge is related to a flow to the compressor being too low, which can cause rapid pulsations in flow.
U.S. Pat. No. 4,464,720 discloses a surge control system which utilizes an algorithm to calculate a desired orifice differential pressure, and which compares the calculated result with an actual differential pressure. Pressure and temperature measurements are made on both the suction side and discharge side of a centrifugal compressor, and thus enter a control system so that the actual differential pressure is substantially equal to the desired differential pressure. A suction temperature of gas entering the centrifugal compressor is measured and used.
However, even with a surge control system damage can occur and the compressor can fail.